The invention relates to a laser device, comprising a radiation source that emits radiation having a spectrum in the form of a frequency comb having a plurality of equidistant spectral lines, and a difference frequency generator that converts the radiation in such a manner that the spectrum of the converted radiation once again has the form of a frequency comb, whereby the frequencies of the spectral lines in the spectrum of the converted radiation are harmonics of a base frequency.
Optical frequency combs have been receiving particularly great attention for the past few years. At the beginning, the main concern was applications of the optical frequency combs in precision spectroscopy. In the meantime, applications in the sector of frequency metrology are also of great interest.
Ultra-short optical pulses in the picosecond and femtosecond range can be produced using mode-coupled lasers. The frequency spectrum of a regular pulse sequence, such as that emitted by a mode-coupled laser, consists of equidistant discrete spectral lines. Such a frequency spectrum is called a frequency comb. The individual spectral lines are very narrow in comparison with their spacing in the spectrum. In this connection, their spacing corresponds to the repetition rate of the pulses, which typically lies in the range between 10 MHz and 10 GHz. Pulse distances between 100 ns and 100 ps correspond to this. The entire spectrum can be many THz in width. In the case of such frequency combs generated by means of mode-coupled lasers, however, it is not the case that the absolute frequencies of all the spectral lines are whole-number multiples, i.e. harmonics of a base frequency. This results from the fact that the electrical carrier field of the radiation experiences a change in phase from pulse to pulse with regard to the envelope of the pulse. This situation is generally referred to with the term CEO phase slip or also, in short form, Carrier Envelope Offset (CEO). The CEO phase slip results from the phase and group velocities of the laser pulse that circulates in the laser resonator, which velocities deviate from one another, and depends on various dispersive and non-linear effects in the laser. Because of the CEO phase slip, the entire spectrum is shifted by a corresponding CEO frequency as compared with the frequency origin. The frequencies of the individual spectral lines of the frequency comb therefore result as a sum of the CEO frequency and a whole-number multiple of the repetition rate of the laser. The CEO frequency is very important in metrology, because the absolute frequencies fn of all the spectral lines are only clearly determined by the indication of the whole-number multiple n of the repetition rate df and of the CEO frequency fCEO.fn=fCEO+n*df 
A laser device of the type stated initially is known from DE 10 2004 022 037 A1, in which device a difference frequency generator is used to generate an optical frequency comb free of CEO. This difference frequency generator converts the radiation of a mode-coupled laser in such a manner that spectral lines are generated, the frequency of which is equal, in each instance, to the difference frequency of two spectral lines of the radiation (which might already have been widened) of the mode-coupled laser. The CEO frequency is eliminated by means of the difference formation, i.e. fCEO=0. The result is a frequency comb free of CEO, in which the frequencies of the spectral lines are clearly defined solely by means of a whole-number multiple of the repetition rate of the mode-coupled laser, because the CEO frequency assumes the value of zero:fn=n*df 
The radiation converted by means of difference frequency generation therefore once again has the form of a frequency comb, whereby the frequencies of the spectral lines in the spectrum of the converted radiation are harmonics of the base frequency.
In the case of the known laser device, it is decisively important that the spectral lines of the radiation of the mode-coupled laser, which lines are subjected to difference frequency generation, have a frequency spacing that corresponds to at least one octave. Actually, a frequency spacing of two octaves is preferred. This is a prerequisite for having the spectrum of the radiation lie approximately in the spectral range of the amplification of the mode-coupled laser once again after difference frequency generation. In the case of the known laser device, it is therefore necessary to first generate a frequency comb that spans more than one octave by means of the mode-coupled laser. Such a spectrum is also called an optical continuum. It is disadvantageous that generation of a continuum that spans more than one octave requires a relatively complicated structure of the previously known laser device. Furthermore, important radiation properties of the frequency comb, such as noise, coherence, etc. are negatively influenced with an increasing widening of the continuum.